Perkin Elmer models 700, 710B and 727 (and probably also model 735) Optical Schematic
Double beam optical null type infrared spectrophotometer
Electronic Schematic page 1 in pdf format
Electronic Schematic page 2 in pdf format
The optical schematic of the Perkin Elmer model 700 (same as model 727) is shown below. The sample beam is just above where it says sampling area and the reference beam is just above that or below where it says optical wedge reference beam. The sector mirror rotates and alternately reflects the reference beam and passes the sample beam. A comb attenuator in the reference beam is connected to the pen servo system and is automatically adjusted by the servo system so that the two beams are kept at the same level as measured by the detector. The pen draws out the spectrum as the chart paper holder moves to the left on the instrument. The grating turns as the chart paper moves. Four filters are included in the model 727 (the one I have) which automatically change at certain points as the spectrum is being scanned (note filter in front of thermopile detector in picture below). These select the desired orders from other spectra orders which are also reflected from the grating. Also, a prism can be used instead of filters to seperate grating orders. Information on the label on the back of the grating says its ruled at 57.6 lines/mm (1463 lines/inch), its blazed at 2.400 ?m, the blaze angle is 6.9? and there is a patent number on it (#2,464,738).
In case you're wondering why they just don't leave the reference beam open all the way and just compare the reference and sample beams electronically, there is one type that does that, the ratio recording spectrophotometer. Also, I've seen densitometer patents that use this arrangement. The disadvantage is that the electronics have to be very linear in the ratio recording system which seems to have been more of a problem to do in the 60's and 70's when these spectrophotometers were made. In the 7xx IR specs, the attenuator is basically a couple wedges stamped out of a piece of sheet metal. The linearity depends on the shape of the wedges
See also
Inside the Perkin Elmer model 700
Infrared Spectrophotometers